The present invention relates to a laser beam printer and, more particularly, to a laser beam scanning position detecting device for determining a print start timing in response to an output signal of a beam detector which is responsive to the passage of a laser beam.
A laser beam printer extensively used today has a photodetector such as a photodiode for determining a write start position in the main scanning position, and determines it by using a positive-going edge of the output signal of the photodetector as a reference. The problem with such a printer is that when the laser power is changed, as often intentionally done to compensate for changes in process conditions, the write start position is shifted. To eliminate this problem, there has been proposed a method which counts the pulse width of an output pulse of a photodetector and determines a write start position on the basis of the count, as disclosed in Japanese patent laid-open publication No. 25363/1986. Alternatively, the intensity of a laser beam may be detected and fed back to a waveform shaping circuit, as taught in Japanese patent laid-open publication No. 233614/1985. Another implementation is such that pulses indicative of a write start position are generated in response to detected intensity of a laser beam, as shown and described in Japanese patent laid-open publication No. 49962/1983. However, the pulse width counting scheme mentioned first cannot attain satisfactory accuracy unless a clock for counting has a frequency which is accuracy unless a clock for counting has a frequency which is high enough to lie in an allowable range with respect to the write errors. The second-mentioned intensity feedback scheme has a drawback that an extra period of time is needed to detect the intensity of a laser beam and the circuitry is complicated. Further, the third-mentioned pulse generating scheme is greatly influenced by an integrating circuit and is complicated in construction.
With a laser printer, digital copier or similar image recorder of the type manipulating a beam-like light source, it is a common practice to fix image forming conditions on a type-of-machine basis at the time of shipment. The image forming conditions include image forming speeds and magnification change ratios (pertaining to the type which changes magnification by changing the frequency of a video clock). However, it is often desired to change, among the same type of machine, the image forming conditions to change the number of recordings, or the change the video frequency or the beam scanning speed to change the magnification. In such a case, correction is needed since the sensitivity of a photoconductive element, for example, changes due to aging although the image forming conditions may be fixed. Among the various image forming conditions, the intensity of a scanning beam may effective for the compensation.
On the other hand, to determine a write start position accurately, it has been customary to provide a beam detector on a scanning line. The output signal of the beam detector is amplified and waveform-shaped to serve as a syncronizing pulse, so that the write start position is determined on the basis of the synchronizing pulse. However, a change in laser power introduces an error in the write start position. The shift of the write start position is not conspicuous so long as an image is monochromatic. However, when it comes to an image recorder of the type manipulating a plurality of beams and, therefore, having a plurality of beam detectors, the error in the write start position occurs with each of the beams. Especially, when a plurality of colors are printed one upon another to form a composite color image, the image quality is critically lowered due to color difference.